Conventional linear power amplifiers (e.g. class A and AB) essentially develop a varying voltage drop across the output driver transistors to reproduce a linear input waveform, which results in power losses at the driver transistors. To achieve improved efficiency, many power amplifiers employ switching power output stages (e.g. class D). For reasons such as this, Class-D power amplifiers have become very favorable in these applications. In audio and servo control applications, a switching power amplifier may have to accept digital input signals, because the incoming signals may be in the digital format (such as in a CD player or in a digital control of a motor).
A Class-D Amplifier uses a technique such as pulse width modulation (PWM) or pulse density modulation (PDM) to convert the incoming input signal, via a sigma delta modulator (SDM) into one or more high frequency pulses having equal width. For example, known class-D switching amplifiers can use a digital Pulse Width Modulator (PWM) to convert an incoming Pulse Code Modulation (PCM) digital signal into PWM signals that can be directly connected to a switching power amplifier. The PWM signals of a Class-D amplifier can drive a switching output stage, such as an inverter or an H-bridge driver (which has three switching levels, i.e., 0, −V, and +V) to drive an external load, such as an external speaker or a servo motor. Optionally, the signal to the load can first be filtered, such as by a low pass filter.
In this case, at any given time the transistors in the output stage are either turned fully on or off, which results in minimal power loss through the transistors of the driver. The voltage delivered to the load is typically controlled by applying pulse-width modulated (PWM) switching waveform to the driver transistor inputs, i.e. a fixed-frequency waveform with a varying duty cycle. The power driver output waveform is smoothed by the low pass filter effect of the load device and/or a power filter network. A drawback to this conventional PWM technique is the presence of a relatively large noise component at the fixed PWM switching frequency.
More recently, instead using a PWM converter, the use of a digital SDM has been proposed in the design of Class-D power amplifiers. For example, in applications such as audio applications, sigma delta modulation can drive an output driver or switching circuit such as an h-bridge. The sigma delta modulation also provides a noise shaping function. These controllers present a pulse-density modulated control signal to the output driver, wherein the number of switching pulses per unit time increases with the magnitude of the input. In this case switching noise is distributed over a broader range of frequencies than in the fixed-frequency PWM technique, resulting in lower peak noise. In addition, because use of SDM causes noise power to be spread over a bandwidth related to the sampling frequency of the SDM, the noise-shaped SDM pulses result in less distortion and noise in the band of interest, as compared with the conventional PWM-based systems. However, both of these conventional systems suffer from load modulation and low power efficiency problems. For example, if an SDM modulator is used in a Class-D power amplifier, there can be switching losses at the power amplifier stage that can reduce efficiency.